1. Field of the Invention
The present invention relates to an optical semiconductor device which is a laser device using a semiconductor, and particularly to a communication semiconductor laser used as a transmission light source of an optical fiber.
2. Description of the Related Art
The Internet becomes established as an infrastructure which is vital for modern society from the business to the home life, and enlarges its range of applications from data communication and blogs to mail order sales, moving pictures, electronic books, SNS (Social Network Service), and the like. With the enlargement of the range of applications, the traffic volume of the optical network supporting the Internet keeps on increasing. Thereby, there has been an increase in demands for optical communication transmission and reception modules which connect between high-speed router devices at a relatively short distance. A transmission rate becomes increasingly high up to 25 Gb/s to 40 Gb/s or more from 10 Gb/s in the related art. At 100 Gb/s which has been recently standardized, four different wavelengths are wavelength-multiplexed at a rate of 25 Gb/s for each channel, thereby realizing large capacity transmission. Currently, an absorption modulator integrated laser is put into a practical laser at a lane speed of 25 Gb/s or more. However, there are strong requests for miniaturization, low power consumption, and low costs, and there are wishes for high speed of a direct modulation laser, a low threshold current, a low current driving operation, and an operation at a wide range temperature from room temperature to high temperature. Particularly, the low threshold current of a semiconductor laser and the high speed in a low current driving condition are important in reducing power consumption of the overall high-speed optical module.
As disclosed in IEEE Journal of Lightwave Technology, Vol. 22, pp. 159 to 165, 2004 by the inventor Kouji Nakahara, et al., in a communication semiconductor laser of a transmission rate of 10 Gb/s or more, a ridge type laser is mainly used in which an InGaAlAs-MQW layer where a relaxation oscillation frequency is high and thus high speed can be realized is applied to an active layer.
A well-known structure which restricts leakage of a current in a semiconductor laser is a burial type (BH: buried heterostructure) laser, and there are a semi-insulating semiconductor BH laser in which a high resistance InP laser is grown up to an upper p type contact layer side, which is disclosed by Takeshi Kurosaki, et al. (IEEE Journal of Lightwave Technology Vol. 14, pp. 2558 to 2566, 1996.), and a pn type BH layer using reverse current blocking of a pn junction as disclosed in JP2010-135506A or disclosed by Y. Ohkura, et al. (Electronics Letters, Vol. 28, pp. 1844 to 1845, 1992.). In recent years, in order to improve characteristics of the ridge type laser, research and development of a BH laser which uses InGaAlAs-MQW in an active layer has been activated, and there has been proposed a semi-insulating semiconductor BH laser disclosed by Koji Otsubo, et al. (IEEE Journal of Selected Topics in QuantumElectronics, Vol. 15, pp. 687 to 693, 2009.) or a pn type BH laser disclosed in JP2008-053649A.
In addition, there are configurations disclosed in JP9-45999A and JP63-169094A as similar laser structures for reducing a leakage current. The configuration disclosed in JP9-45999A has a structure in which an Fe—InP layer is buried in both sides of the ridge structure in a stripe shape. In JP63-169094A, current constriction is realized by putting disordering the vicinity of an active layer outside a ridge using impurities.